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I want to use a line voltage 240v thermostat to keep my electric hot water tank just from freezing....
Some of the models reviewed online show a pattern of cheaply made garbage that burn out after a short time etc.
Can I get a recommend on a rugged mechanical unit that is less expensive than the electronic models but will last and be reliable??thanks

Even the heavy-duty mechanical thermostats (e.g., https://www.supplyhouse.com/Honeywell-T6051A1016-Heavy-Duty-Line-Voltage-Thermostat-46-to-84F) are limited to 8-10 amps at 240V. A water heater CB is typically 30A, so you may have to combine a thermostat with a heavy-duty power relay capable of switching 30A (e.g., P&B 655-T92S7A22-240). Then you could use a cheap thermostat plus a good relay (under $20) to do the job. And, you could then separate the thermostat from the water heater location if you want. Make sure the thermostat is capable of switching the relay's coil voltage and current.

Basically, use the relay to switch the WH's power, and the thermostat to switch the relay's coil power. It would be simpler if the WH, thermostat, and relay coil were all 240V devices, but any good electrician, handyman, or ham radio guy/gal should be able to do it.

IMHO, you need to switch both legs of the 240VAC supply to the WH, so you'd need a DPST relay or thermostat. I couldn't find such a thermostat, so chose to go the relay route, which also turned out to be cheaper. Quick and dirty, use a Cadet BTF2 thermostat ($20) to control the P&B relay ($12). Everything's 240VAC. Hardest part would be enclosing everything in a box of some kind and actually running the cables nicely.

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lowest setting is a useless 90F. THis setup is just to keep wh from freezing and use minimul power.THe thermostat will have to have a probe and be attached to the skin of the wh and be under the insulation jacket.
I think I can wire the set up...good practice anyway..but most 30 amp relays that are reliable are about 30$,no? Is thsat Pand B for 12 bucks reliable??Maybe a 40 amp would be prudent..?

when a relay uses a 110v AC to 24vDC but controls a 240v AC large load ,what is that called?

Click to expand...

I don't know that it's called anything special, but I'm not a control systems engineer. Relays can be specified by contact form, contact current rating, switching voltage, and a raft of other things. P&B is now another name, apparently (TE Connectivity Potter & Brumfield), but P&B used to be a very reliable name. If you truly need to monitor the tank temperature with a probe under the insulating bracket, it gets more complicated, and it may be hard to find a mechanical thermostat with a remote probe like that. But if the insulation is baggy enough, you could probably stuff something like the Cadet in there, just be careful not to let anything interfere with the thermostat contacts.

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I will need to replace my Lochnvar 40 ga unit soon...its 12 years old. I was thinking of the Rheem 20 ga
..I dont need that much hot water and its about 350$. Would that model have a thermostat that lets
you select a temp of 40 F..??? What other units would have that?

Well-Known Member

What about the connecting piping? Aren't they subject to freezing as well?

Not knowing all the details of concern, etc. I must ask, why not just drain the tank?

Or calculate what the actual standby loss in electricity cost would amount to. All the trouble, cost and chance taking may not be worth the potential savings. I calculated this once many years ago and the unit I was looking at amounted to about 100 watts a day. In fact I bet there is info available some where listing that info based on differential temp and insulation.

Well-Known Member

If your tank is located somewhere where freezing of the tank is a possibility, you need to properly winterize the entire setup or as Diehard said, you'll lose the pipes as well. Your hot water heater has very good insulation from heat loss your pipes will freeze well before the tank. If this is a vacation place or something like that you need to plan better.

Hardcore DYI-er

If this house has a furnace then set the thermostat for 40 degrees. If no furnace then a full winterization is needed. Drain all pipes and the water heater and pour RV antifreeze in all the traps (including toilets).